The invention relates to a shock absorber for slowing down the descent of a control rod within a thimble tube in a nuclear reactor and particularly to an improved thimble tube construction.
As well set forth in various prior art patents such as U.S. Pat. Nos. 3,562,109, 3,773,617, 3,980,519, 3,992,255 and 4,028,180, it is conventional to provide control rods for adjusting the operating conditions of nuclear reactors. The core of the reactor typically contains a grid arrangement of thimble tubes which contain reactor coolant and are interspersed among the fuel rods. Positioned in telescopic relationship with each thimble tube is a reactor control rod which contains a neutron-absorbing material. The control rod is displaceable in a known controlled manner or by gravity between an upper position in which the absorbing portion is located outside the intense neutron flux zone of the reactor and a lower position in which the absorbing portion is located within the intense neutron flux zone of the reactor.
Generally, the control rods can be lowered to slow down the reaction to varying degrees. For safety purposes, the control rods are also mounted in such a manner that all of them may be dropped simultaneously by gravity in order to effect an immediate shutdown of the reactor. In order to cushion the impact in such a "scram" situation, a shock absorbing means is commonly built into the thimble tube. Perhaps the simplest form of shock absorber is provided by forming a necked-in area in the thimble tube so that the associated control rod can drop quickly to the necked-in region and then be resisted in its further movement by the fact that the liquid displaced as the rod drops into the closed end thimble tube must move through the narrow space between the rod and the necked-in wall portion. Where a single restriction is provided in the thimble tube, rather substantial resistance forces are developed which are undesirable. In order to reduce these forces, the aforementioned U.S. Pat. No. 3,562,109 proposes the use of a plurality of necked-in regions which cause the control rod to slow its descent in a number of steps rather than very abruptly. Thus, the reaction forces are absorbed relatively uniformly over an extended distance of travel. Although the aforementioned multiple restriction construction provides adequate adsorption of forces, it is rather expensive to construct and the elongated necked-in regions provide some opportunity for foreign particles to become jammed where they could interfere with the free descent of the rods.